1. All speedos are small. They're not shorts or swimming briefs, and therefore are more comfortable and look better.

2a. Speedos don't need to be extremely tight(bulge). That causes weals. It's uncomfortable for your dick and/or balls and that's not necessary (unless you like that). What you want is your dick and balls to have some space to move around. You want a bulge.

2b. Need enough space. Especially after you finished swimming and the shrinkage from the cold water goes away, you need enough space in your speedos to be comfortable. (people who like squeezing their junk in an uncomfortable way excluded). You want a bulge.

3. Sagging is normal. If the speedos are so tight that they don't sag (the cock is pressed against the body, the balls are squeezed), and the speedos never need to be pulled up a bit, then they are not the right size. Probably too small/tight. Go out and buy new ones. (see 2.)

4. You don't care, nobody really cares. You don't care if there are some people who might think you're showing off. If you'd care about that, you'd wear shorts. Most people won't think anything of it anyway. Speedos are normal swimwear and you can see them around in all sorts.

5. Never buy black speedos. Then you might as well wear shorts. (see 4.)

6. Do not tie the string. Unless you're a fast speed swimmer it's not necessary to tie the string. You can lose the string all together, or just let it hang out. When you're on the beach, at the pool, the string is meant to be kept untied (see 2, 3, 4.)